The present invention relates to a process for the manufacture of a corrosion resistant chromium steel.
Austrian Pat. No. 277,301 discloses a nitrogen containing steel which has a high yield strength and good toughness characteristics. It contains up to 0.6% carbon, 5 to 40% chromium, up to 30% manganese, up to 5% molybdenum, up to 20% nickel, 1.5 to 5% nitrogen, the remainder iron, and has an austenitic structure. The nitrogen content is introduced into the steel by initially adding nitrogen containing iron-chromium or iron-manganese alloys to the melt, then introducing gaseous nitrogen into the melt or into the slag. The teaching of Austrian Pat. No. 277,301 is based on the long held understanding that in austenitic chromium-nickel and chromium-manganese alloys, nitrogen increases the austenite stability, and that in semiferritic and ferritic chromium steels containing more than 18% chromium, the presence of nitrogen results in the formation of austenite and in an increase in the proportion of transformable structures. With respect to austenite stability, 0.1% nitrogen may replace 2% nickel (see E. Houdremont, Handbuch der Sonderstahlkunde [Handbook of Special Steels], 1956, pages 1327-1331).
However, the production of a heat-resistant, corrosion-resistant, nitrogen containing chromium steel having a primarily ferromagnetic structure from a chromium steel having a predominantly ferromagnetic structure has never before been achieved. It would have been of interest to the industry, however, because chromium steels having ferromagnetic structures have good high temperature characteristics. For example, corrosion resistant chromium steels, containing 12 to 18% chromium, 0.5 to 1% manganese, 0.05 to 1.2% carbon, 0 to 1% silicon, 0 to 2.5% nickel, 0 to 1.3% molybdenum, 0 to 2% vanadium, 0 to 0.3% aluminum, the remainder iron, exhibit the following material characteristics after annealing or age hardening:
yield strength R.sub.p0.2 =250 to 600 N/mm.sup.2 PA1 tensile strength R.sub.m =450 to 950 N/mm.sup.2 PA1 elongation at rupture A.sub.5 =20 to 12% PA1 contraction Z=60 to 40%
The ferromagnetic structures of these corrosion resistant chromium steels, in the annealed state, consist of ferrite or of both ferrite and perlite and, in the age hardened state, consists of both ferrite and transformation phases, of transformation phases, or of martensite.
Compared to the austenitic chromium-nickel steels, chromium steels having a ferromagnetic structure are distinguished by having superior strength characteristics and by having very good resistance to stress crack corrosion. Even at temperatures of up to 400.degree. C., the strength characteristics of ferritic chromium steels having a ferromagnetic structure lie far above the values for austenitic chromium nickel steels, while their characteristic deformation values lie noticeably below the values for austenitic steels. However, beginning at about 450.degree. C. the high temperature stability of ferritic chromium steel drops considerably because of embrittlement which begins in this temperature range. Use of such steels for continuous operation is therefore limited to temperatures below 300.degree. C. (see Werkstoff kunde der gebrauchlichen Stahle [Materials of Commonly Used Steels], Part 2, published by Verlag Stahleisen mbH, Dusseldorf, 1977, page 165).